In Search of a Consistent Solution to Allocation of Joint Production

Three consistency problems are identified that arise when partitioning a product system with joint production according to an allocation key, such as revenue or mass of the joint products, namely: lacking consistency of rationales and procedures; lacking consistency of monetary, mass, and energy balances in the partitioned product systems; and lacking consistency of results across model resolution and classification of the intermediate flows. Different solutions to these consistency problems are described, including the attempt of ecoinvent to solve the third consistency problem with a system model that uses revenue allocation at the point of substitution. The problems with the different practical implementations are described. For each of the three consistency problems, a solution is proposed and combined into a single consistent solution. The consistency of rationales and procedures is ensured by asking only one question at a time and performing a separate allocation and calculation for each question. The problem of maintaining monetary, mass, and energy balances is solved by a generalized allocation correction. The identified problem with consistency of results across model resolution and classification is solved by redefining the point of substitution. It is described what consequences the solutions will have if their results are misused for decision making that will shift demand between products.